This invention relates generally to multi-compartment electrodialysis methods for recovery of acid values from metal salts, especially multivalent metal salts that form substantially insoluble precipitates (hydroxides or oxides) upon addition of base, such as calcium salts of gluconic acid derivatives. These inventors discovered that the use of an inexpensive acid as a source of protons, particularly in a four compartment configuration electrodialysis cell, allows the conversion of such multivalent metal salts into a desirable acid product and a soluble multivalent salt by-product.
The recovery of acid values from multivalent metal salts by electrodialysis has not been practiced because the membranes of electrodialysis cells can be subject to fouling with insoluble hydroxide precipitates when multivalent metal salts are split into the acid and base components. Four compartment electrodialysis, also termed double decomposition or metathesis, is a useful technique for recovering the acid values from such salt streams. Metathesis has been used to recover iminodiacetic acid, a key intermediate in the production of glyphosate herbicides, from its sodium salt, but has not been applied to the recovery of valuable acids from multivalent metal salts. Also, the four compartment process has not been applied to the recovery of valuable gluconic acid derivatives, such as 2 Keto-L-gulonic acid, (a key intermediate in Vitamin C production) from multivalent metal salts of the acid.
In cases where the acid form of a multivalent metal salt is more valuable than the salt, it is desirable to convert the salt to the acid. Conventional methods for recovering the acid value from a salt have included precipitating the cation by addition of an acid such as sulfuric acid that forms an insoluble multivalent metal salt. Cation or anion exchange resins have also been used to exchange the multivalent cation for proton to produce the desired acid. However, in some cases these processes do not produce the acid in adequate purity or concentration, or are not economical. Salt splitting electrodialysis processes with bipolar membranes are useful for producing concentrated and purified acids from salts since the anion is transported across anion permeable membranes. However, salt splitting electrodialysis also forms the corresponding base from the salt, and hence the bipolar and/or cation membranes can foul with insoluble precipitates when the cation is a multivalent metal. Multi-compartment electrodialysis allows the recovery of acid values and avoids these potential fouling problems.
The requirement for the recovery of acid values from a multivalent metal salt can be demonstrated in the production of ascorbic acid from sugars via fermentation. The fermentation converts the sugar to a 2-keto-L-gulonic (hereinafter KLG) acid salt, which is a key intermediate in the production of ascorbic acid. KLG must be converted to the free acid form before chemical conversion to ascorbic acid. The fermentation cannot be run to form free KLG acid because KLG acid is a fairly strong organic acid (pKa=2.7). Therefore, aqueous solutions of KLG acid are too acidic to allow fermentation to proceed. A suitable base is added to the fermenter to maintain a near neutral pH and form a salt of KLG. In the case of the bacterium used to form KLG, the most suitable base is calcium hydroxide. KLG productivity, yield, titer, and cell viability are all reduced significantly when monovalent bases, such as sodium, potassium, or ammonium hydroxide are used to control pH during fermentation. Therefore, the preferred product of the fermentation is a solution of calcium KLG. A conventional approach to acidification of CaKLG2 is to add sulfuric acid to equivalence, thereby forming KLG acid and calcium sulfate. Calcium sulfate is sparingly soluble, so calcium sulfate is filtered off and residual soluble calcium sulfate may be removed by cation and anion exchange steps. This process produces large amounts of calcium sulfate waste that must be landfilled for a fee, and has a relatively high capital equipment cost due to the need for cation and anion exchange and evaporation of large amounts of water from the relatively dilute broth.
Four compartment electrodialysis (metathesis) is an attractive alternative because it would produce concentrated and purified KLG acid directly from the fermentation broth and a soluble calcium salt by-product of potential economic value. Accordingly, four compartment electrodialysis methods have been developed to permit recovery of the acid value from a multivalent metal salt.
It is a principal object of this invention to provide a novel multi-compartment electrodialysis (metatheis) method for the recovery of acid values from multivalent cation salts that form insoluble precipitates upon addition of hydroxide.
It is a further object of this invention to provide a novel electrodialysis method for converting CaKLG2 to concentrated and purified KLG acid and a soluble calcium salt by-product. These objectives can be met particularly well with a four compartment electrodialysis method comprising the steps of:
a) introducing a first feed solution into a first electrodialysis feed compartment, the feed solution comprising a multivalent metal salt in water, such as CaKLG2;
b) introducing a second feed solution in a second electrodialysis feed compartment consisting of a strong acid solution in water, such as an aqueous solution of hydrochloric acid;
c) transporting an anion of the multivalent metal salt, such as the KLG anion in the first feed compartment across an anion exchange membrane and into an acid product compartment;
d) forming the corresponding acid product (KLG acid) in the acid product compartment by addition of proton from the second electrodialysis feed compartment solution adjacent to the KLG acid compartment. The proton from the strong acid solution in the second feed compartment is transported across a cation exchange membrane into the acid product compartment;
e) removing the multivalent metal cation, such as calcium from the first feed compartment by transporting across a cation exchange membrane and into the by-product calcium salt compartment, and
f) forming the corresponding soluble calcium salt by-product, such as calcium chloride, by transporting the anion of the strong acid from the second feed compartment across an anion exchange membrane into the by-product calcium salt compartment.
In this embodiment, the multivalent metal salt is converted into a more valuable acid product and a soluble salt of the multivalent cation as a by-product. This is accomplished by providing a strong acid as a second electrodialysis feed stream that provides protons to form the desired acid product and anions to form the soluble multivalent metal salt by-product. The by-product may be potentially valuable product equal to or greater than that of the added strong acid, so that the by-product enhances the overall economic advantage of the metathesis process. Examples of such co-products include calcium chloride and calcium nitrate.